What's wrong with my kitten?

I just got a little bob-cat kitten last a week ago. She's a little scrawny one like her half sister was as a baby. They both came from the same mother. I've noticed that almost on a day to day basis, she has a clump of poop stuck on the stub where the tail should be. I try to clean it as much as possible, but it keeps reappearing. I just lost 2 cats within a year and a half... Not even a year of each, from the fatal and incurable disease Bobcat tick disease. I don't want this one gone either. Help?? Please???